1. Field of the Invention
The present invention relates to a position controller usable with a shape-memory-alloy actuator adapted to move a movable member using a shape-memory alloy in a biased manner, and capable of controlling a position of the movable member. The present invention also relates to a driving mechanism and an image pickup system equipped with the position controller and the shape-memory-alloy actuator.
2. Description of the Related Art
A shape-memory alloy (hereinafter referred to as “SMA”) has a crystal structure called “austenite phase (parent phase)” at a temperature relatively higher then a transformation temperature, and a different crystal structure called “martensite phase” at a temperature relatively lower then the transformation temperature. In ordinary metal materials, if a given external force is applied thereto, they will never return to their pre-deformed shapes. In contrast, even if an SMA in the martensite phase is deformed due to a given external force applied thereto, the deformed SMA can be heated up to the transformation temperature or more to induce a phase transformation from the martensite phase to the austenite phase, so that the deformed SMA is recovered to its original (i.e., pre-deformed) shape. By utilizing this characteristic, an actuator using an SMA (i.e., shape-memory-alloy actuator) has been developed.
FIG. 13 is a graph showing a relationship between a temperature and a resistance of a shape-memory alloy in an isolated state (i.e., an unbiased state without a biasing force applied thereto). In FIG. 13, the horizontal axis represents a temperature T (displacement D), and the vertical axis represents a resistance R.
As shown in FIG. 13, an SMA in an isolated state has a characteristic CX, wherein the resistance R of the SMA increases along a gradual curve with a rise in the temperature T of the SMA so that it reaches a maximum resistance value Rmax at a given temperature value TRmax, and then turns to decrease so that it reaches a minimum resistance value Rmin at a given temperature value TRmin, whereafter the resistance R turns to increase again (TRmax<TRmin, Rmax>Rmin). Particularly in the range of the maximum resistance value Rmax to the minimum resistance value Rmin, the resistance R of the SMA decreases at a rate proportional to the temperature T of the SMA, while exhibiting high linearity. In addition, a displacement D of the SMA induced by electrical heating corresponds to the temperature T of the SMA, and therefore the characteristic CX illustrated in FIG. 13 can be considered as a relationship between the electrical heating-induced displacement D and the resistance R of the SMA.
As a position controller for such a shape-memory-alloy actuator, there has been developed a type utilizing the SMA's property where the resistance R changes linearly relative to the displacement D, as disclosed, for example, Japanese Patent No. 2769351 (hereinafter referred to as “D1”).
A position controller disclosed in the D1 comprises: an actuator adapted to operate based on a displacement of a shape-memory alloy element; driving means adapted to selectively heat and cool the shape-memory-alloy element; comparison means adapted to activate the driving means based on a deviation between a displacement of the actuator and a target displacement value; resistance detection means adapted to detect a resistance R of the shape-memory-alloy element; storage means adapted to obtained and store therein a maximum resistance value Rmax and a minimum resistance value Rmin of the shape-memory-alloy element in advance of initiation of a position control; and displacement calculation means adapted to calculate the displacement of the actuator based on an output of the resistance detection means and information stored in the storage means. In this position controller disclosed in the D1, even if the resistance of the shape-memory-alloy element is fluctuated due to environmental changes, fatigue thereof or other factor, the fluctuation in the resistance can be automatically compensated so as to adequately perform the position control while maintaining a high degree of accuracy. In the D1, with a view to compensating a position of the actuator to perform the position control with further enhanced accuracy, the position controller is additionally provided with a position sensor.
An actuator intended to repeatedly produce movements in response to temperature rise and fall is required to have a two-way (i.e., two-directional) function responsive to the temperature transition or shift. However, although an SMA is recovered to a memorized shape according to heating, the resulting recovered shape of the SMA will be retained even if it is cooled. That is, an SMA in an isolated state has only a one-way (i.e., one-directional) function. Therefore, in one aspect, a shape-memory-alloy actuator is required to have a biasing member operable to apply an external force (biasing force) for deforming the SMA in a second direction different from the one, i.e., first, direction, after shape recovery.
Additionally, in the position control, it is necessary to define a reference position (i.e., a reference value of the position control) for determining a current (i.e., actual) position, and figure out at least one relationship between a displacement (position) and a resistance of the SMA, as a precondition to defining the reference position. It is contemplated to utilize, as this relationship, a displacement (position) of the SMA at a maximum resistance value Rmax, and/or a displacement (position) of the SMA at a minimum resistance value Rmin. However, an actual shape-memory-alloy actuator equipped with the biasing member has a configuration which makes it impossible or difficult for the SMA to exhibit the maximum resistance value Rmax and the minimum resistance value Rmin which otherwise appear in the SMA in the isolated state. Moreover, the structure provided with a position sensor as in the D1 causes an increase in the number of components, which leads to difficulty in downsizing.